The designers of insulation systems for large high voltage AC dynamoelectric machines have faced a constant challenge over the last century, and that is to provide a machine which will operate at higher and higher voltages whilst keeping the layers of insulating material on the critical electrical conducting parts to a minimum thickness. To meet the challenge, designers have utilized mica in a variety of forms from large flake dispersed on a backing material, to the product known as mica paper which is a product made from tiny mica flakes which are incorporated into a product which nearly resembles paper, and in fact is made by a process which is very similar to a process used for making paper from pulp fibers.
Mica by its very nature has physical properties which make it intractable for use in an insulation system. However, its superior corona breakdown resistance has provided the incentive that insulation system designers needed to overcome the obstacles created by its unattractive physical qualities. Today, mica paper enjoys an unchallenged position as being one of the most corona discharge resistant materials known to mankind.
Mica paper has an inherently low tensile strength and the tiny mica flakes composing the paper tape used in most mica paper insulation tend to flake from the body of the tape as it is wound on the winding elements, which will subsequently become a part of an AC machine. As a result, insulation designers currently bond the mica paper to another insulating medium, traditionally glass fibers which will improve the tensile strength of the mica paper and the backing tends to prevent the shedding of mica flakes from the mica tape during a winding operation.
At the present time, a composite insulation which has exceptional insulation qualities and good corona discharge resistance is a CR KAPTON.RTM. (trademark of DuPont) insulating film, which is used as a backing on a mica paper, glass fiber composite tape. The addition of enhanced corona resistant materials yields an overall insulation system which is electrically more robust than standard systems. Experimentation has shown that by strategically locating the highly corona resistant materials in the high electrically stressed locations, the resultant hybrid system becomes dielectrically superior (higher volt per mil capability) than either a fully corona resistant system or a standard system. The resulting system has the added advantage of being less costly than a fully corona resistant enhanced system.